At powerup the module waits for 5 seconds for an audio signal with firmware on the output jack. If nothing is input it starts normal operation. Otherwise it updates its firmware. No programmer required.

Here is a link for downloading the HEX file for the TinyAudioBootloader:

This HEX uses PB1, pin6 for the programming audio signal input.
The pin should be lifted to Vcc/2 with a resistor divider.

Set the fuses to 0xE1, 0xDD, 0xFE when programming.

More info on the TinyAudioBootloader can be found here:https://github.com/ChrisMicro/TinyAudioBoot

If you are unable to program the ATtiny85 yourself you can also have the preprogrammed chip here:

http://www.dspsynth.eu

If nothing is plugged in to the CV1/CV2 jack the knobs works as parameter dials. If they are jacked the knobs work as attenutators for any signal plugged in.

The CV3 jack can be used to input both digital and analog signals and output a digital signal.

All inputs are protected against euro voltages and the power has a reverse protection diode.

The Paperface line is of course open-source so the disclaimer:

My work on these free synthesizers is based on donations from people.
If you find the code useful, please consider a $3 donation to keep future developments open source.

The Phase Distortion Oscillator

The PD oscillator works like the classic Casio CZ synthesizer. In essence it is really a form of FM. At the time Yamaha had patent on their FM (or PM) synthesis so Casio had to come up with a workaround.

It works by modulating a cosine wave by a modulation index and enveloping it with a slope to remove the nasty level jump.

So it really simulates a saw wave run through a resonant lowpass filter.

The modules CV1 input is Pitch 1V/Octave and the CV2 input is the modulation index, 0.125 – 16x

The CV3 input is an audio gate input that allows you to run it with for example the Beatstep Pro without having a VCA module for some Acid patterns.

Here is the source-code for the PD module code.
The code is the same used in my CZ1 chip and the CZ-VCO module.

// (*) All in the spirit of open-source and open-hardware
// Janost 2017 Sweden
// The 3HP Paperface Euro Modules
// https://janostman.wordpress.com/the-3hp-paperface-euro-modules/
// Copyright 2017 DSP Synthesizers Sweden.
//
// Author: Jan Ostman
//
// This program is free software: you can redistribute it and/or modify
//it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.